Comment on “Correcting the Dst index: Consequences for absolute level and correlations” by A. Karinen and K. Mursula

Already Bartels, Mayaud, and others have stressed that a geomagnetic activity index ideally should monitor or measure a distinct physical process or relationship [realizing that the process may be complex and in actuality compound]. The classical example of an index that fails this criterion is the “daily range” mixing the effect of solar EUV and of the solar wind. In designing the K-index, Bartels attempted [and largely succeeded] to separate out the solar EUV effects as manifested in the regular diurnal variation and thereby eliminate them from the index bringing it closer to a measure of the efficiency of the coupling between the magnetosphere and the magnetized solar wind on a time scale comparable with the time it takes the solar wind to “pass” the extended magnetosphere.